Photolithographic masking and etching is widely understood because of its extensive use in the fabrication of integrated circuits. In such processes, a semiconductor wafer is coated with a thin layer of photoresist material and is exposed to actinic light through a patterned photomask. After development, the photoresist coating itself acts as a mask to permit selective processing, such as selective etching, of the wafer.
It has been found that the same process is useful for making optic fiber connectors from silicon wafers. Each connector comprises a silicon chip having grooves on opposite sides which are used to contain optic fibers and to align the connector. Since the alignment of optic fibers for lightwave transmission is extremely critical, it is important the the support grooves be accurately located to within tolerances that are typically less than one micron. For this reason, in the fabrication of such connectors, it is critical to align accurately photomasks on opposite sides of a semiconductor wafer that has been coated with photomas material.
The U.S. Pat. No. 3,963,489, of K. H. Cho, issued June 15, 1976, is an example of prior art teachings of methods of aligning photomasks on opposite sides of a semiconductor wafer. The Cho technique uses indexing indicia on corresponding portions of the two photomasks that overlap the wafer contained between them. By properly aligned these indicia, one aligns the mask patterns of the two masks with respect to the intervening semiconductor wafer.
A problem with using a scheme such as this for aligning masks with submicron tolerances is that a sufficiently high-power microscope for making the alignment will normally have a smaller depth of field than the separation of the two masks. Thus, the operator cannot see both alignment indicia simultaneously during the alignment operation. One way of meeting this problem would be to align the two masks while they are in close contact and then separate them to incorporate the wafer. In practice, it has been found to be extremely difficult, if not impossible, to make the required mechanical separation while maintaining the required close tolerances.